1. Field of the Invention
This invention relates to an image display apparatus comprising a component capable of image display by an applied voltage and particularly, to an improvement in an image display apparatus comprising a driving semiconductor array integrated with a display substrate.
2. Description of the Prior Art
Image display apparatuses, in particular, liquid crystal image display apparatuses where a liquid crystal is used as a component capable of displaying images by an applied voltage, have generally the following features which active display does not have.
(1) Passive display and a very little consumption of electric power.
(2) The apparatuses can be driven by a low voltage.
(3) The apparatus can be in a form of panel.
(4) A layer scale display is possible.
This type of image display apparatus has been recently studied and developed.
For utilizing the above features, as to (1) above, a reflection type of structure which does not need an illuminating source consuming a lot of power, and as to (2) above, a liquid crystal of twisted nematic structure and a mode of a low voltage operation utilizing guest host effect.
As to (3) and (4) above, the apparatus works satisfactorily for display handling conventional relatively small amount of information, but a system capable of displaying a lot of information has been recently desired.
For example, a display apparatus capable of matrix driving many image elements as shown in U.S. Pat. No. 3,824,003 has recently attracted public attention.
A semiconductor array for driving a display apparatus as mentioned in the U.S. Patent has a substrate "S" (as shown in FIG. 1), gate lines 101a, 101a' . . . composed of a thin transparent or metal conductive film which overlie the substrate, an insulating layer I and a semiconductor 102 laminated thereon. Source lines 103, 103' . . . composed of a conductive film contacting the semiconductor 102 are oriented at right angles to the gate lines 101a, 101a' . . . Drain electrodes 104, 104', 104", 104'" . . . which are segment electrodes are formed around intersections of gate lines and source lines.
The above mentioned semiconductor is shaped in the form of a thin film and is a driving switching element represented by thin film transistor (TFT).
Display apparatuses similar to the above ones are disclosed in IEEE trans. on Electron Devices ED-20, p. 995 (1973). A driving switching element of the display apparatus is illustrated in FIG. 2A. On a substrate S (for example, glass) is formed TFT in a form of matrix with 2-10 lines/mm. The TFT comprises gate lines 201a and 201a' formed on a substrate S, gate electrodes 201, 201', 201", 201'" mounted on the gate lines, an insulating layer I laminated on the gate electrodes, thin film semiconductors 202, 202', 202", 202'" formed on the gate electrodes with an intervening insulating film I, source lines 203, 203', contacting one end of the semiconductors, and drain electrodes 204, 204', 204", 204'" contacting the other end of the semiconductors.
A counter electrode as shown in FIG. 2B is arranged facing to the substrate provided with the above mentioned trasistor array and thus an image display apparatus is constructed.
In display apparatuses, when, in particular, a liquid crystal layer is used, various states of orientation of liquid crystal molecules, and various optical detecting means such as polarizing plate, .lambda./4 plate, reflection plate and the like may be optionally selected depending upon a display mode selected, for example, dynamic scattering mode (DSM), twisted nematic (TN) and the like or the type of apparatus, for example, a transparent type, a reflection type and the like (In FIG. 2B, 207 stands for a counter substrate such as glass, 208 a counter electrode, 209 an insulating film, 210 a spacer and 211 a liquid crystal layer.).
The driving method is, in general, as shown below.
For example, image signals are applied to gate lines 201a, 201a', and driving voltages are applied to source lines 203, 203' by scanning (only while a signal is applied to the gate line) and thereby, the channel between sources 203, 203' and drain 204", 204'" becomes conductive at the selected intersection.
And there is formed an electric field between the drain electrode and the counter electrode 208 and arrangement of liquid crystal molecules in liquid crystal layer 211 changes to effect display.
When a display wherein a plurality of gradations or a high speed response is required, such as display of television image signals, is effected by the above mentioned display apparatus, it is known that the operation characteristics of the display cell is very sensitive to the thickness of liquid crystal layer.
Therefore, it is necessary for obtaining good gradation or high speed response that the thickness of liquid crystal layer is made as thin as possible (for example, from several microns to 10 microns) over a certain area or more (for example, 10 cm square or more) and is made uniform.
It is known that for the purpose of keeping the thickness of liquid crystal layer constant, inert members of a certain size are incorporated in the liquid crystal, or point-like matters such as resin in a form of parallel line, circle, triangle or polygon are formed over almost all surface of the substrate.
However, such methods firstly disturb molecular arrangement of liquid crystal and lower the "appearance". In addition, it is considerably difficult to incorporate uniformly inert members in a liquid crystal. In addition, driving unstability often occurs by a short-circuit between the electrodes.
Further, since the above mentioned display cell is a passive type, there are the following disadvantages, that is, it is necessary to illuminate the display surface by an external light. Semiconductors such CdS, CdSe and the like used for TFT have a remarkable photoconductivity and therefore the external light for observation sometimes makes the operation characteristics unstable.
For solving such problem, there may be used a method of employing Te, a method of laminating a (multi-layered) reflection increasing film on the semiconductor and the like. However, in view of poisonous property, less availability of the material, complicated steps and the like, those methods are not always practical.
In addition, from a practical point of view, it is considerably difficult to manufacture an image display apparatus capable of satisfying simultaneously the above mentioned conditions (1)-(4). For example, for the purpose of effecting display by a predetermined mode such as DSM (dynamic scattering mode), twisted nematic (TN), deformation of vertical aligned phase (DAP), hybrid and the like, it is necessary to bring preliminarily liquid crystal molecules to an appropriate orientation state, and various methods are employed.
The representative methods are a method of vapor-depositing SiO or SiO.sub.2 obliquely as to the substrate (oblique vapor deposition method), a method of rubbing a glass substrate, and the like.
The above mentioned oblique vapor deposition method has the following disadvantages. This method requires a large scale apparatus such as vacuum deposition apparatus, can handle only a small amount of the material per one operation and therefore the productivity is low. Further, when the thin layer is prepared, microscopic pinholes are liable to form, and electrochemical reactions occur through the pin holes.
In case of a method of rubbing a substrate, there are disadvantages that rubbing of the substrate does not always give a sufficient orientation of liquid crystal molecules and the resulting orientation state is not durable.
For the purpose of eliminating such disadvantages, an organic high polymer layer is laminated on segment electrodes on a substrate and rubbing is applied to the high polymer layer, but when the cell is sealed with a sealing material such as glass frit upon fabricating (usually heated to 300.degree. C.), the organic high polymer is subjected to heat deterioration resulting in an insufficient orientation state.
On the other hand, as a substrate material for the cell, it is desirable to use an inexpensive glass such as alkali glass, but when such a glass is used for cells, there occurs driving unstability after a short period of time of operation. That is, when segment elements are disposed on a glass substrate, chemical decomposition reactions of liquid crystal are accelerated by impurity ions present in the glass. Or when a protecting layer composed of an organic high polymer is disposed on segment electrodes on a substrate so as to prevent such reactions, if the organic high polymer layer is made so thick that a sufficient effect can be obtained, the driving voltage applied to the electrode does not sufficiently act on liquid crystal, and the threshold voltage tends to increase and sharpness tends to be lowered or non-uniform orientation tends to occur.
Still another disadvantage of conventional image display apparatus is as shown below. When the above mentioned image display cell is made in a form of reflection type, a transparent type of a display cell can not be constructed if semiconductor 102 (in FIG. 1) is made of an opaque material because, for example, in case of FIG. 1, semiconductor 102 covers substantially the whole surface of substrate 102 in case of a display apparatus comprising a driving switching element as shown in FIG. 1. Since most of semiconductor 102 used here is opaque, a reflection type is employed.
Further, most of semiconductor 102 used here is photoconductive, and therefore, in case of a liquid crystal cell which displays images by using ambient light or illumination there is used a reflective member for the purpose of achieving the driving stability by means of a light intercepting electrode and for the purpose of effectively observing optical change of liquid crystal.
Therefore, drain electrodes 104, 104', 104", 104'" are metal electrodes, and there occurs a mirror reflection since the thickness of cell is kept constant.
For the purpose of efficiently catching the mirror reflection as an optical change, there has been conventionally used dynamic scattering mode (DSM). However, the mirror surface structure results in formation of mirror images of lightening, external light for illumination, wall, furniture, a face of an observer on the display surface and therefore, the display effect is disturbed. In order to avoid such drawbacks, an image display apparatus is partly practically used which is operated in such a way that the image is observed from an oblique direction only and a hood of a drak color is mounted in the direction of the normal reflection.
However, this type of image display apparatus can be used for displaying numbers which usually occupy only a relatively small area, but is not preferred for a display of a brood area such as a flat panel television since it is natural to see the display from the front and such observation from the front is easy for seeing the images.
In addition, DSM has the following drawbacks, that is, DSM has a relatively high driving voltage, the driving life is shorter than that of the other electric field effect type mode, and color display is difficult. Therefore, it could be proposed to use the other mode for the cell of such mirror type, but the drawbacks of the mirror surface electrode structure is enhanced and can not be decreased even when TN mode or guest host mode is used.
In order to solve such problem, Japanese Patent Laid Open No. 54-37697 proposes a method that a substrate at the back side is treated to form a rough and uneven surface and it is used as a reflection electrode scattering light. The resulting structure has various drawbacks, that is, disconnection of the TFT structure, fluctuation of characteristics and non-uniform thickness of the liquid crystal cell.
When the display picture element electrode 204 of the driving switching element as shown in FIG. 2A is made of a reflective material such as metal, there is a drawback as mentioned above, i.e., mirror image reflection.
According to this structure, drain electrode 204 contacts semiconductor 202 and therefore this structure contacts partly an opaque portion, but it is possible from the point of view of operation that the substrate, the insulating layer and pair plates of the capacitor are composed of transparent materials. Therefore, in case of a liquid crystal display cell in the form of TFT, the display portion excluding the semiconductor portions may be composed of two substrates of a transparent material. In such a display apparatus, when it is reflection type, for example, a scattering reflection plate is fixed to the back surface of the cell, a cell of TN structure which is employed in usual clocks and portable calculators can be easily used.
However, this structure also has some drawbacks. For example, it is desirable for display that the pattern of semiconductor 202 portion is smaller than that of the display portion. Therefore, it is required that a finer pattern processing of the semiconductor is made as compared with the pattern processing of the display pattern.
When a transparent electrode is used as a drain electrode 204, there is sometimes a problem as to an ohmic contact with semiconductor 202 If a structure, i.e., semiconductor/ohmic contact member/transparent electrode, is employed so as to solve the problem, the number of step disadvantageously increases.
Apart from above, when a known transparent electrode such as indium oxide, tin oxide and the like is fabricated according to conventional methods, a high temperature step has adversely a thermal effect on the resulting semiconductor 202. In particular, when amorphous silicon is used as the semiconductor, such a light temperature step destroys the desirable characteristics.
In view of the foregoing, the liquid crystal display apparatus using TFT as shown in FIG. 2A has various problems.